Demineralized bone matrix (DBM) is widely used in the repair of pathologies associated with skeletal defects and periodontal diseases. This material is typically produced from cortical bone of long-bones (chiefly those bones found in the legs and arms of human cadaveric donors) by cutting the shafts of these long-bones into small chunks (1-4 mm) using methods well-known in the field. The resulting pieces and chunks of bone are subsequently cleaned and grinded into a finer bone powder. The resulting bone powder is typically in the about 125 to 1000 micron particle size ranges. The bone powder may be demineralized by exposure to dilute (normally 0.4 to 0.6 N) hydrochloric acid, organic acids, calcium chelating agents, etc. as is known in the art. For example, U.S. Pat. Nos. 5,275,954; 5,531,791; 5,556,379; 5,797,871; 5,820581; 6,189,537; and 6,305,379 describe methods of demineralizing bone material and are hereby incorporated by reference in their entirety. This ground demineralized bone matrix material has been called demineralized freeze-dried bone allograft (DFDBA), demineralized bone allograft (DBA), demineralized bone matrix (DBM), and demineralized bone (DMB) and is currently produced by a number of for profit and not-for-profit companies for use in orthopaedic, spinal fusion, and periodontal applications.
The use of DBM in the formation of new bone has been assessed using in vivo (usually a mouse or rat implant system), in vitro (cell culture or extraction and quantitation of bone forming molecules reportedly present in bone), and in situ (where the formation of new bone in patients has been assessed during clinical applications) applications. Methods of assessing this new bone formation and the effects of the demineralization process on new bone formation by DBM are described in Zhang et al., “A quantitative assessment of osteoinductivity of human demineralized bone matrix.,” J. Periodontol. 68:1076-1084 (1997) and Zhang et al., “Effects of the demineralization process on the osteoinductivity of demineralized bone matrix,” J. Periodontol. 68:1085-1092 (1997). An in vitro assessment of the ability of DBM to induce cells towards an osteoblastic phenotype has also been described (Wolfinbarger and Zheng, “An in vitro bioassay to assess biological activity in demineralized bone,” In Vitro Cell Bio. Anim. 29A:914-916 (1993)).
DBM is assumed to form new bone when implanted in animal models via an endochondral pathway. The implanted DBM is presumed to cause mesenchymal stem cells (typically undifferentiated fibroblasts) to migrate towards the implanted biomaterial(s). This induced chemotaxis results in cells infiltrating the implanted DBM biomaterial(s) where they are induced to undergo phenotypic changes from a fibroblastic cell phenotype to a chondrocyte phenotype and eventually to an osteoblast cell phenotype. These induced phenotypic changes have been reported to be due to the action(s) of one or more small molecular weight proteins falling in the TGF-β family commonly referred to as bone morphogenetic proteins (BMPs). As the change in cell phenotypes occurs, the proliferative potential of the cells declines. For example, the population doubling times increases from approximately 12 hours to approximately 40 hours. As a result, the cells synthesize and secrete collagens and other matrix-forming proteins/glycoproteins laying down a cartilagenous matrix and finally an osteoid-like matrix, which if left implanted in the animal long enough, can be shown to mineralize. This process is analogous to the formation of new bone. If the implanted materials lack the cell-inducing protein factors, only providing an environment suitable for cellular infiltration and cellular proliferation and differentiation, the implanted materials are deemed to be osteoconductive. If the implanted materials possess the cell inducing protein factors and provide an environment suitable for cellular infiltration and cellular proliferation and differentiation, the implanted materials are deemed to be osteoinductive. If the implanted materials already contain cells suitable for new bone formation, such as autogenously transplanted bone, the materials are deemed to be osteogenic.